1. Field of the Invention
The present invention relates to a method for operating a moving hearth reducing furnace, in which iron oxide agglomerates incorporated with a carbonaceous material are reduced to iron.
2. Description of the Related Art
A typical method for preparing reduced iron is a MIDREX process. In this method, a reducing gas, such as natural gas, is blown into a shaft furnace through a tuyere. The reducing gas flows in and comes contact with iron ore or iron oxide pellets filled in the furnace. Thus, iron oxide is reduced in a reducing atmosphere in the furnace to form reduced iron. This method using a large amount of expensive natural gas, however, inevitably results in high production costs.
Recently, processes for making reduced iron using inexpensive coal in place of the natural gas have attracted attention. For example, U.S. Pat. No. 3,443,931, which is hereby fully incorporated by reference, discloses a process for making reduced iron including pelletizing a mixture of powdered iron ore and coal and reducing iron oxide in a hot atmosphere. This process has some advantages: use of coal as a reducing agent, direct use of powdered ore, a high reducing rate, and ready control of the carbon content in a product.
In this process, a given amount or depth of pellets or briquettes of iron oxide agglomerates incorporated with a carbonaceous material (hereinafter referred to as simply "agglomerates") is fed into a moving hearth reducing furnace, such as a rotary hearth furnace. The contents are moved and heated by radiant heat in the furnace. Thus, iron oxide is reduced with the incorporated carbonaceous material to form reduced iron. The reduced iron is discharged from the moving hearth of the furnace by a screw of a discharging apparatus. The process of U.S. Pat. No. 3,443,931 is shown in FIG. 12. In FIG. 12, the screw 1 of the discharging apparatus is supported by an elevator 3 and a bearing 4, comes into contact with a moving hearth 2 by its own weight, and rotates to discharge the reduced iron from a discharging port 25.
When the agglomerates are fed into the moving hearth furnace, parts of the agglomerates are pulverized by rolling, friction or dropping shock and the iron oxide powder is deposited on the moving hearth. As shown in FIG. 13, the iron oxide powder moves towards the screw 1 and is reduced to metallic iron powder 26. The metallic iron powder on the rotating hearth is squeezed into the furnace face by the screw and is deformed to elongated metal powder 28 (see "Initial Forming Stage of Iron Sheet" in FIG. 13). The elongated metal powder 28 squeezed into the furnace is barely oxidized in a reducing atmosphere. Thus, the elongated metal powder gradually grows by the pressure of the screw 1 and becomes an iron sheet (see "Iron Sheet Forming Stage" in FIG. 13).
In a hearth surface of the rotary hearth furnace, there is a temperature difference of at least 300.degree. C. between the heating and reducing region and the feeding region in the furnace. This temperature difference is transferred to the iron sheet 29 by the rotation of the rotary hearth, and thus the iron sheet 29 repeatedly expands and shrinks. As a result, cracks form in the iron sheet 29. When pressure by the screw 1 is applied to the cracks of the iron sheet 29, a warp forms in the iron sheet 29. The iron sheet 29 having a large warp catches on the screw 1 and is detached from the hearth (see "Detachment of Iron Sheet" in FIG. 13). A grown detached iron sheet 29 inhibits discharge of reduced iron 10 by the screw 1 and causes problems such as shutdown (see "Shutdown due to Detachment of Iron" in FIG. 13).
Furthermore, pits are formed on the moving heath during the formation and detachment of the iron sheet. Since the agglomerates are deposited on the pits, the depth of the fed agglomerates is not stable, and the agglomerates are not uniformly heated. Accordingly, the quality of the reduced iron is deteriorated.